Download Module 7: Atmosphere As inhabitants of the planet Earth, we live at

Module 7: Atmosphere
As inhabitants of the planet Earth, we live at the bottom of an ocean. This ocean is the
atmosphere, the layers of the air that surround the earth.
The atmosphere affects almost everything we do. It directly influences everything in our
environment. Changes in the atmosphere cause changes in our weather and climate. This,
in turn, influences such important things as:
1. the development of soils;
2. the growth of vegetation (and thus animal life that feeds on plants);
3. the growth and development of minerals and fuel deposits.
The atmosphere is a chemical mixture. It contains gases, liquids, and solids. Like all
gases, air possesses the property of mass. That is, it occupies space and is influenced by
the force of gravity.
Scientists who study the atmosphere are called meteorologists. They have found that the
atmosphere is striated, that is, it has several different layers. Each layer differs from the
others in:
1. temperature,
2. chemical composition,
3. physical properties.
A layer of air called the atmosphere surrounds the earth. It extends a few hundred miles
above the surface of the planet and is held in place by gravity. Air contains several gases,
which we cannot see. However, we need to breathe one of these gases oxygen in order to
live. Air also protects us from the strongest rays of the sun called ultraviolet light.
The atmosphere is composed of many gases. It is 78 percent nitrogen, 21 percent oxygen,
and 0.033 percent carbon dioxide. There are also tiny amounts of argon, helium, krypton,
xenon, and hydrogen. Hydrogen and oxygen combine to form water vapor. Each of these
gases is important. People need carbon dioxide and nitrogen so that they can grow. Water
vapor becomes rain or snow to keep the ground wet for plants to grow.
The atmosphere is also filled with tiny particles of dust and smoke. Many people who
have allergies sneeze just from breathing air with too much dust in it. We will see later in
this lesson that the dust helps with the formation of clouds. First, though, we will see how
the gases that make up the air help people, animals, and plants live.
Troposphere
The lower atmosphere is called the troposphere. This is the layer nearest the earth. Its
depth varies from 8 kilometers (4.8 miles) over the poles to about 17.6 kilometers (11
miles) over the equator. At low altitudes, up to about two miles, there is enough air for
people and animals to breathe and live. The troposphere extends upward from the ground
(sea level) for about ten miles (16 kilometers). The average depth of the troposphere is
11.2 kilometers (7 miles).
Most of our weather phenomena are located in this layer. This includes such things as
clouds and warm and cold air masses. Almost all of the water in the entire atmosphere is
found here. All of the clouds that we see in the sky are in the troposphere. This is the
warmest part of the atmosphere.
The troposphere is heated by radiation from the sun, both directly and by heat radiating
from the earth. It is warmer nearer the earth’s surface. As altitude increases (the higher up
you go), temperature decreases. For every kilometer in altitude, the temperature drops
6.5oC (or 3.5oF for every 10,000 feet in altitude).
We live in the troposphere, at the bottom of the earth’s atmosphere. We are under the
pressure of all the air above us. An instrument called a barometer measures
atmospheric pressure. If you listen to the weather report on television, you may hear the
weather mention the barometric pressure. When air pressure drops, it often means that a
storm is coming.
The Stratosphere
The next layer is colder, the average temperature is -40oC. There is very little water and
this air is less dense in this layer. The stratosphere extends above the troposphere to an
average of 50 kilometers (30 miles).
Two important features associated with the stratosphere are:
1. The jet stream is an enormous current of air. It’s between the troposphere and the
stratosphere. These winds blow from west to east at 200 miles per hour.
2. The ozone layer is found within the upper region of the stratosphere. It is made
up of a form of oxygen called ozone (O3). The ozone protects life on earth from
the sun’s harmful ultraviolet rays. Ozone is slightly different from the oxygen we
breathe and would burn our throats if we tried to breath very much of it. Ozone
absorbs invisible ultraviolet light that would other wise be very dangerous to life
on the planet. It does this by reacting with the radiation to form a stable form of
oxygen (O2). This keeps the radiation from reaching the earth. The reaction gives
off heat. Thus the air at the top of the stratosphere is warmer than at the lower
levels.
The Mesosphere
The mesosphere is above the stratosphere and extends up to fifty miles (80 kilometers)
above the earth’s surface. The temperature here falls to around -100oC as you reach the
top of the mesosphere. Near the top, the temperature stops falling.
The Thermosphere
The thermosphere extends from 80 to 960 kilometers (50 miles to 600 miles) above the
earth. It contains most of the ionosphere, which is not a layer of the atmosphere but is
very important to us.
Ultraviolet and X-ray energy from the sun electrify the molecules in the air. The atomic
particles thus become electrically charged particles called ions. These ions form layers at
different layers in the ionosphere. The layers reflect radio waves back to earth. This
makes it possible to bounce waves off the ionosphere and back anywhere on the globe.
The Exosphere
Here the atmosphere is so thin that it barely exists. There are very few air molecules in
this region. The composition is mostly hydrogen and helium gases. Beyond the exosphere
is outer space---vast emptiness.
The Composition of Air
When meteorologists describe air, they’re referring to air in the lowest layer, the
troposphere. From here we can go on to say that the atmosphere (or air in the
atmosphere) is mostly a mixture of gases. What is called pure air is a mixture of four
major gases, plus small traces of other gases. It’s colorless, odorless, and tasteless.
The Four Major Gases:
Nitrogen (N2): Nitrogen is the most abundant gas in the air. But it doesn’t combine easily
with other substances. It is important for the development of plant and animal tissues.
Nitrogen compounds are also used in fertilizers.
Oxygen (O2): Oxygen is used by animal life in the respiration process. It does combine
easily with other elements. In fact, many of our life processes involve oxygen
compounds. Two of the compounds are water (H2O) and carbon dioxide (CO2).
Argon (Ar): Argon is chemically inactive, but is of lesser value to us. Its most common
use is in electric light bulbs. The bulbs are filled with argon because it won’t combine
chemically with the metal filaments in the light bulbs.
Carbon dioxide (CO2): There is not much carbon dioxide in the air. However, it is one of
the most important gases in the atmosphere. It’s used by green plants during
photosynthesis to produce simple sugars. Most forms of life would cease to exist on our
planet if carbon dioxide were not present. This is because the green plants that use carbon
dioxide are a source of food for other life forms. The plants give off oxygen into the
atmosphere as a waste product of photosynthesis.
Water Vapor (H2O): This is the gaseous form of water and can be found in the
atmosphere. It helps to trap heat and keep the earth warm. It is found in clouds, fog,
snow, hail, and rain.
The amount of matter (solids and liquids) in the air varies from place to place on the
earth. The percentage of each of the four major gases will remain in the same ratio
throughout the atmosphere. Scientists believe the proportion has been essentially the
same for tens of thousands of years. That is because plants and animals recycle the O2 and
CO2.
The O2 and CO2 cycle:
CO2 is given off as a waste product from animals during respiration. Plants then use the
CO2 for photosynthesis. This use of CO2 by the plants removes CO2 from the atmosphere
and fixes it in simple sugars. The simple sugars are used for energy in animal cells. Plants
give off O2 in the photosynthesis process as a waste product. Animals use the O2 to live
and complete the energy transfer in their cells during respiration. The animal cells give
off CO2 as a waste product of respiration. The c returns to the atmosphere for another trip
around the cycle.
Nitrogen is recycled in the same manner. Plants and animals use nitrogen to build cells.
When the plants and animals die, small bacteria break down the cells and the nitrogen is
returned into the soil or air.
SUMMARY
Air Pressure: Physical Properties
The atmosphere has matter in it; therefore it has mass. Remember matter is defined as
anything that has mass and occupies space. This mass pushes down on the surface of the
earth (actually the atmosphere is pulled toward the center of the earth by gravity). The
force of this mass is called air pressure. An average pressure of 14.7 ponds is exerted on
each square inch of the earth.
Air Pressure is defines as:
14.7 lbs/in2 760 mm Hg
760 torr
101325 Pascals
1 atmosphere
As you go up into the atmosphere, say going up into the mountains or on an airplane, the
air pressure decreases. The opposite is true for decreasing in altitude; air pressure
increases. Remember the lower you are in the atmosphere; the more of the atmosphere is
pushing down on you.
Air pressure depends on how dense the air is. Density is the quantity of mass per unit of
volume. Thus, the more molecules of air per given volume, the greater the density.
Density is a ratio of mass per volume. Therefore 14 grams of air in 1 liter of space would
have a density of 14 g/liter.
Three things influence air density.
Gravity: The Law of Gravity states that objects closer to each other attract each other
more. That is, air particles in the troposphere will be pulled more by gravity than particles
farther in the mesosphere. More air molecules will collect in a certain volume near the
earth’s surface. Higher up, fewer molecules will collect in a given volume. Density
decreases with altitude.
Temperature: Heat energy causes molecules to move faster and spread out from each
other. This means a cubic foot of warm air has fewer molecules than a cubic foot of cold
air. If fewer molecules are present, the density is less. Therefore warmer air is less dense
and has less air pressure and will rise over cooler air.
Water Vapor: Water vapor lowers the density of air. A cubic foot of water vapor weighs
less than a cubic foot of nitrogen or oxygen gas. Thus moist air is less dense than dry air.
The average air pressure at sea level and 0oC is 14.7 pounds per square inch. The air
doesn’t always exert this pressure everywhere. The pressure changes daily.
Meteorologists calculate the air pressure from using with a barometer. They can
calculate the change in pressure by using the differences in two readings. High pressure
areas will “flow” to low pressure areas. The movement of air caused by the flow from
high pressure areas to low pressure areas is called wind.